Address a timing side channel whereby it is possible to determine some

[openssl.git] / crypto / ec / ecdsa_ossl.c

diff --git a/crypto/ec/ecdsa_ossl.c b/crypto/ec/ecdsa_ossl.c
index 6ff5a462d34563fe3dd2edfee474de2ad4fde829..ef912822ee9acfa208ef4692ffb7412f71df196b 100644 (file)
--- a/crypto/ec/ecdsa_ossl.c
+++ b/crypto/ec/ecdsa_ossl.c
@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2016 The OpenSSL Project Authors. All Rights Reserved.
+ * Copyright 2002-2017 The OpenSSL Project Authors. All Rights Reserved.
  *
  * Licensed under the OpenSSL license (the "License").  You may not use
  * this file except in compliance with the License.  You can obtain a copy
@@ -20,7 +20,7 @@ int ossl_ecdsa_sign(int type, const unsigned char *dgst, int dlen,
                     const BIGNUM *kinv, const BIGNUM *r, EC_KEY *eckey)
 {
     ECDSA_SIG *s;
-    RAND_seed(dgst, dlen);
+
     s = ECDSA_do_sign_ex(dgst, dlen, kinv, r, eckey);
     if (s == NULL) {
         *siglen = 0;
@@ -41,6 +41,7 @@ static int ecdsa_sign_setup(EC_KEY *eckey, BN_CTX *ctx_in,
     EC_POINT *tmp_point = NULL;
     const EC_GROUP *group;
     int ret = 0;
+    int order_bits;
 
     if (eckey == NULL || (group = EC_KEY_get0_group(eckey)) == NULL) {
         ECerr(EC_F_ECDSA_SIGN_SETUP, ERR_R_PASSED_NULL_PARAMETER);
@@ -77,6 +78,13 @@ static int ecdsa_sign_setup(EC_KEY *eckey, BN_CTX *ctx_in,
         goto err;
     }
 
+    /* Preallocate space */
+    order_bits = BN_num_bits(order);
+    if (!BN_set_bit(k, order_bits)
+        || !BN_set_bit(r, order_bits)
+        || !BN_set_bit(X, order_bits))
+        goto err;
+
     do {
         /* get random k */
         do
@@ -89,7 +97,7 @@ static int ecdsa_sign_setup(EC_KEY *eckey, BN_CTX *ctx_in,
                     goto err;
                 }
             } else {
-                if (!BN_rand_range(k, order)) {
+                if (!BN_priv_rand_range(k, order)) {
                     ECerr(EC_F_ECDSA_SIGN_SETUP,
                              EC_R_RANDOM_NUMBER_GENERATION_FAILED);
                     goto err;
@@ -100,13 +108,19 @@ static int ecdsa_sign_setup(EC_KEY *eckey, BN_CTX *ctx_in,
         /*
          * We do not want timing information to leak the length of k, so we
          * compute G*k using an equivalent scalar of fixed bit-length.
+         *
+         * We unconditionally perform both of these additions to prevent a
+         * small timing information leakage.  We then choose the sum that is
+         * one bit longer than the order.  This guarantees the code
+         * path used in the constant time implementations elsewhere.
+         *
+         * TODO: revisit the BN_copy aiming for a memory access agnostic
+         * conditional copy.
          */
-
-        if (!BN_add(k, k, order))
+        if (!BN_add(r, k, order)
+            || !BN_add(X, r, order)
+            || !BN_copy(k, BN_num_bits(r) > order_bits ? r : X))
             goto err;
-        if (BN_num_bits(k) <= BN_num_bits(order))
-            if (!BN_add(k, k, order))
-                goto err;
 
         /* compute r the x-coordinate of generator * k */
         if (!EC_POINT_mul(group, tmp_point, k, NULL, NULL, ctx)) {
@@ -182,7 +196,7 @@ static int ecdsa_sign_setup(EC_KEY *eckey, BN_CTX *ctx_in,
         BN_CTX_free(ctx);
     EC_POINT_free(tmp_point);
     BN_clear_free(X);
-    return (ret);
+    return ret;
 }
 
 int ossl_ecdsa_sign_setup(EC_KEY *eckey, BN_CTX *ctx_in, BIGNUM **kinvp,
@@ -221,6 +235,12 @@ ECDSA_SIG *ossl_ecdsa_sign_sig(const unsigned char *dgst, int dgst_len,
         ECerr(EC_F_OSSL_ECDSA_SIGN_SIG, ERR_R_MALLOC_FAILURE);
         return NULL;
     }
+    ret->r = BN_new();
+    ret->s = BN_new();
+    if (ret->r == NULL || ret->s == NULL) {
+        ECerr(EC_F_OSSL_ECDSA_SIGN_SIG, ERR_R_MALLOC_FAILURE);
+        goto err;
+    }
     s = ret->s;
 
     if ((ctx = BN_CTX_new()) == NULL ||
@@ -321,7 +341,7 @@ int ossl_ecdsa_verify(int type, const unsigned char *dgst, int dgst_len,
 
     s = ECDSA_SIG_new();
     if (s == NULL)
-        return (ret);
+        return ret;
     if (d2i_ECDSA_SIG(&s, &p, sig_len) == NULL)
         goto err;
     /* Ensure signature uses DER and doesn't have trailing garbage */
@@ -332,7 +352,7 @@ int ossl_ecdsa_verify(int type, const unsigned char *dgst, int dgst_len,
  err:
     OPENSSL_clear_free(der, derlen);
     ECDSA_SIG_free(s);
-    return (ret);
+    return ret;
 }
 
 int ossl_ecdsa_verify_sig(const unsigned char *dgst, int dgst_len,